When an oil and/or gas well, or a portion of the well, ceases to become economically viable, that well or portion of the well may be abandoned. Abandoning a well involves sealing the intervals of the well to prevent the migration of oil, gas, brine and other substances into freshwater and preventing the migration of water or other contaminants into the oil and gas reservoirs.
A wellbore is very often drilled to depths many thousands of feet from the surface. The resulting disruption of geologic formations can cause contamination of otherwise useful fluid reserves when a fluid from one formation flows through the wellbore to a different formation. Well owners and operators have long known of these potential risks, but have increasingly become aware of the changes that can occur within a wellbore over very long periods of time. Past preferred methods of properly abandoning and preventing leakage between fluid reserves included placing cement plugs within the wellbore, across and on top of hydrocarbon bearing or aquifer zones. That cement placement forms a long-term seal and isolation of the formations of interest. The interval to be cemented may be up to several hundred feet in length.
The wellbore, however, may include fissures running on the outside of the outermost casing. Leakage between formations may thus occur on the outside of the casing even if the inside of the casing is sealed by a cement plug. The industry has increasingly become aware of the need to remove the casing entirely from within wellbore. When the casing is completely removed, the cement plug directly contacts the formation. Using existing equipment, operators generally remove the outermost casing using mechanical milling techniques; however, there are many drawbacks to the milling process. The operation is slow and may take a month or more to complete. The contaminated metal cuttings of the casing must be returned to the surface for processing and disposal. The milling drill must be large and powered by heavy rigs at the surface of the wellbore. Furthermore, if there is any interior casing or production tubing strings left in the wellbore, those must be removed before any drilling of the external casing. Therefore, a need exists for a long-term seal of a wellbore while minimizing time and financial resources used in pulling casing and/or production tubing strings from a wellbore and milling the outermost casing.
Alternatively to milling the casing, some abandonment projects consider perforation of the casing to be adequate. Operators typically use explosive perforating techniques to form holes in the casing throughout the zone(s) to be plugged. As known in the art, a perforating gun containing a series of shaped charges is lowered into the wellbore and the charges are ignited through electrical or mechanical means. The perforations provide a flow-path for cement between the interior of the casing and the annulus.
While perforation is typically easier than complete removal of the casing, perforation has several drawbacks. It is often difficult to achieve an adequate flow-path between the interior of the casing and the annulus, in some instances. Inadequate or inconsistent explosive perforation through the casing prevents cement from adequately flowing between the interior of the casing and the annulus. Under those conditions, the cement may not completely seal the annulus. These problems have been addressed, in some instances, by implementing a “cement squeeze,” into the targeted area. A cement squeeze is a technique in which the cement is highly pressurized as it is forced into the wellbore. The pressurization is believed to ensure that cement fills any and all cracks in the casing or surrounding formation. A cement squeeze may be especially employed in wellbores which have multiple layers of piping and/or casing. That is, the inner tube string(s) may be perforated with a perforating gun and cement squeezed into the area. The cement is forced through the perforations in the inner tube string and fills the annulus between the inner tube string and the outer casing layer.
In a properly formed cement squeeze, cement hardens on both sides of the casing, ostensibly sealing that zone of the wellbore. Long term studies of wellbores have revealed, however, that after a few years the casing itself starts to deteriorate. In many circumstances, a deteriorating casing leaves fissures through which fluids may leak. Even a properly implemented cement squeeze does not address the problem of casing deterioration. Furthermore, cement squeeze techniques typically still require heavy equipment capable of producing the high pressures.
Therefore, a need exists for a wellbore sealing and isolation technique that does not require milling, explosive perforation, or tubing string extraction.
A need exists for sealing and isolation techniques which do not require a drilling rig, or a high pressurizing rig, to be transported to the wellbore site.
A need exists for sealing and isolation techniques that are not susceptible to fissures caused by deterioration of the casing after a cement plug has been established, which can lead to contamination issues.
Given the drawbacks associated with mechanical milling and with the explosive perforation, there is a need in the art for additional techniques for removing sections of casing or for creating adequate flow paths within the casing to facilitate abandonment operations.